The present invention relates to an X-ray exposure mask for forming microscopic patterns on a high density semiconductor device.
With a recent trend toward a high packing density of a semiconductor device, micro-patterning is normally used for fabricating the high density semiconductor device and advances are now being made in this area due to development of an X-ray exposing technology. The X-ray exposing technology is necessary to form a microscopic pattern onto a semiconductor wafer by exposing an X-ray beam, having a wavelength of several angstroms, to an X-ray resist material coated on the semiconductor wafer. When a light beam or an electron beam is used for patterning a microscopic pattern on the order of a submicron, for example diffraction, interference, and scatter of the light beam, or the electron beam, produces problems which reduce the resolution of the microscopic pattern. These problems are eliminated by applying the X-ray exposing technology.
Because the wavelength of an X-ray beam is very short, a conventional light exposure mask cannot be used in X-ray exposing technology. A X-ray exposure mask having a special structure for the X-ray beam is used, and the X-ray exposure mask will be called "an X-ray mask" hereinafter. Thus, the special mask is used for patterning the microscopic pattern by using the X-ray beam, however, the patterning is carried out in the same way as patterning for an ordinary semiconductor device. Moreover, many of the same patterns for fabricating a plurality of semiconductor chips are formed on a semiconductor wafer by using a patterning apparatus called "a stepper", and furthermore a plurality of patterns, having different patterns respectively, are stacked by repeating the patterning.
FIG. 1 is a schematic diagram of a plurality of chip patterns 101 formed on a semiconductor wafer 100. In order to stack a plurality of patterns having different shapes respectively, a plurality of X-ray masks respectively having different patterns are necessary.
FIG. 2 is a schematic diagram of an example of such an X-ray mask. Reference numeral 200 represents an X-ray mask and reference numeral 201 is a membrane supported by a ring frame 202, which will be simply called "ring" 202 hereinafter; and, the membrane 201 will be explained later. The reference numeral 203 is a pattern for the semiconductor chip formed on the membrane 201.
As an example, in FIG. 2 four patterns 203 are provided having the same shape and size, respectively. X-ray patterns obtained from the patterns 203, by using the X-ray beam, are sequentially exposed on the semiconductor wafer 100 as shown in FIG. 1. The hatched patterns 103 within a circle 102 in FIG. 1 indicate the patterns being exposed sequentially. The pattern 203 in FIG. 2 is used to form the chip pattern and several patterns, different from the pattern 203, are usually stacked in order to complete a semiconductor chip pattern. That is, the semiconductor chip patterns 101 are formed on the semiconductor wafer 100 by using additional masks each having the same structure as that of the mask 200, but having different mask patterns 203 which complete the semiconductor chip patterns by stacking the X-ray patterns 101 of the different mask patterns 203 at the same position. Particularly, this stacking of patterns 203 must be done accurately because each pattern 203 to be stacked is a microscopic pattern. Further, positioning of the X-ray masks 200 is very important and currently a light beam is preferred for positioning the X-ray masks 200.
Reference numeral 204 depicted in FIG. 2 indicates four mask positioning marks for such positioning. Dots 104 depicted in FIG. 1 are wafer positioning marks previously provided to the semiconductor wafer 100. Sequentially exposures of X-ray patterns by the different X-ray masks 200 onto the semiconductor wafer 100 are executed through positioning by sequentially aligning the mask positioning marks 204 on the X-ray masks 200 to the wafer positioning marks 104 on the semiconductor wafer 100, by using the light beam.
For executing positioning using the light beam, optical mask alignment technology utilizing the diffraction of the light beam is currently the most advanced technology, from the point of view of accuracy.
This optical mask alignment technology has been explained in detail in "Application of Zone Plates to Alignment in Microlithography" by M. Feldman et al. (J. Vac. Sci. Technol., Vol 19, Vo. 4, Nov./Dec. 1981). As explained in this document, the X-ray mask 200 must have the structure of a window, through which a light beam passes, around each mask positioning mark 204 in order to apply the optical mask alignment technology. Namely, a part of an incident light beam is reflected by a mask positioning mark 204, on the X-ray mask 200, however the remainder of the incident light beam reaches the wafer positioning mark 104 as previously marked on the semiconductor wafer 100, by passing through the window provided at the periphery of the mask positioning mark 204 and then is reflected by the wafer positioning mark 104. The light beam reflected by the wafer positioning mark 104 again passes through the window and is gathered in a light condensing system along with the light beam reflected by the mask positioning mark 204. The mask positioning mark 204 and the wafer positioning mark 104 are imaged in the light condensing system and compared with each other. Next the position of the X-ray mask 200 is moved, relative to the position of the semiconductor wafer 100 until the images are superimposed on each other. Thus, the positions of the X-ray mask 200 and the semiconductor wafer 100 are aligned.
Thus, the light beam is used for the mask alignment in the patterning process and the X-ray beam is used for the patterning in fabrication of the high density semiconductor device. Therefore, the material of the X-ray mask 200 must be translucent not only to the X-ray beam but also to the light beam because making only the window portion of the X-ray mask 200 translucent to the light beam is difficult in view of the cost and cost efficient is an advantage of the present invention.
As a substrate of the X-ray mask 200, a membrane 201 of a material having a good transmission characteristics for the X-ray beam, such as boron nitride (BN) having a small atomic number, is used. Namely, in the case where conventional substrates such as glass or quartz are used for the membrane 201, the X-ray beam cannot be used because these substrates have a low transmission factor for the X-ray. A BN membrane is translucent to the light beam as well as the X-ray beam because of being as thin as several microns. However, the membrane has low mechanical strength and cannot be independently maintained flat therefore the membrane is adhered to a ring frame 202 for support.
FIGS. 3(a)-3(f) are schematic diagrams which indicate a process for fabricating an X-ray mask 200 of the prior art. These figures may be understood, as the showing where the sectional view taken along the line A-A' of FIG. 2 is indicated, moreover a 205 wide space between two patterns 203 A-A, related to the line A-A' is eliminated.
The prior art X-ray mask 200 has been fabricated by the following process. (1) The BN membrane, is deposited, by a chemical vapor deposition (CVD) method, on a silicon wafer (Si wafer) which is adhered to a ring frame 3, which will be called "ring" hereinafter, made of material, such as PYREX or ceramics, having a small thermal expansion coefficient. (2) A side, on which the BN membrane, is not deposited, of an internal part of the Si wafer looking from the ring 3 is removed by etching. Thus, the BN membrane 1 is supported flat by a ring 3, as shown in FIG. 3(a). (3) Titanium, tantalum - gold - tantalum, gold, or platinum, etc. is sputtered to a thickness of about 400-500 angstroms on the BN membrane 1. The sputtered layer is a plating base 4, which is a base layer used for easily plating metal on the BN membrane 1. (4) A photo resist material of a polymeric resin, or an electron beam (EB) resist 5 is coated on the plating base 4 as shown in FIG. 3(b). (5) The photo resist or the EB resist 5, is exposed or written, by a light beam or an electron beam, respectively, and then developed, thereby forming a photo resist pattern or an EB resist pattern called a stencil 6 is formed, as shown in FIG. 3(c). (6) Metal of gold (Au) is plated on the plating base 4 by using the plating base 4 as an electrode, and an aperture of the stencil 6 is filled up with the Au layer 7 as shown in FIG. 3(d). The Au layer 7 is an absorber for the X-ray beam. (7) After the aperture of the stencil 6 is filled up with the Au layer 7, the stencil 6 is removed by an ashing process. (8) The plating base 4 appears, after the stencil 6 is removed, is removed by a reactive ion etching (RIE) method, using carbon tetrafluoride (CF.sub.4) gas or a sputter etching method, using argon (Ar) gas. FIG. 3(e) indicates a state where the stencil 6 and the plating base 4 thereunder have been eliminated. The stencil 6 and the plating base 4 are eliminated in order to make the vicinity of the positioning marks 204, provided to the X-ray mask 200, translucent on the light beam for executing the mask alignment. (9) Finally, as shown in FIG. 3(f), whole faces of the Au layer 7 and the membrane 1 are coated with a protective thin layer 8, which is translucent to the X-ray beam and the light beam, and thereby a conventional X-ray mask 200 has been completed.
However, the X-ray mask of the prior art explained above requires the eliminating process of the stencil 6 and the plating base 4 thereunder, which produces a problem in that grooves which are formed among the Au layers 7, after eliminating the stencil 6 and the plating base 4, are too narrow. Thus the grooves are easily destroyed by the successive processing, so that some of the patterns provided by the grooves are deformed or eliminated, and some of the Au layers 7 will be peeled. Explaining in further detail, the Au layer 7 is required to have a height of 0.5-1 microns in order to absorb the X-ray, and the width of stencil 6 is 1 micron or less, for example 0.1-0.2 microns, so that patterns of the Au layer 7 have a high aspect ratio, resulting in the fabricating structure being easily damaged.
In view of solving the above problem, an X-ray mask has been developed by, wherein a material having a high transmission factor for the X-ray is used for the stencil, the plating base is thin enough to allow the X-ray to pass through and the stencil is not required to be eliminated (Japan Published Unexamined Patent Application No. 193031 (1982)). According to this method, polycrystalline silicon, silicon nitride, silicon oxide or a composite material thereof is used for the stencil. However in selecting the material to transmit the X-ray beam, the transmission factor for the light beam is not considered. Accordingly, the positioning marks which should be translucent to the light beam, in the vicinity thereof, cannot be provided on the X-ray mask.
Proving the mask positioning marks in an area far from the patterns for the chips, such as an area near 3, must be avoided because the ring positional errors due to thermal expansion existing between positioning marks and the chip patterns increase. The spaces 205 and positions for the mask positioning marks 204 in FIG. 2 are determined considering the above. However, since it is difficult to make only the vicinity of each mask positioning mark translucent to the light beam due to the cost, as mentioned before, the stencil 6 must be eliminated in the fabrication process of the prior art X-ray mask.